The present invention relates to a method of repainting a reflector array antenna, especially a reflector array antenna used on board a geosynchronous satellite.
Array antennas form one or more radiation patterns using a set of individual sources whose signals are combined by a digital or analog beamforming network. Array antennas can therefore form a plurality of patterns simultaneously, i.e. multibeam coverage, by applying a plurality of different feed laws. Multibeam coverage is frequently used in telecommunications, especially in systems using geosynchronous satellites.
Given the very high altitude of geosynchronous satellites, the multibeam coverage of the array antennas used on board them is obtained by using very narrow beams, typically having a beam width of the order of one degree. For patterns that are this directional, small amounts of depointing can cause strong variations in the power radiated in a given direction. Consequently, it is important for the beams to be pointed very accurately. At present, a pointing accuracy of the order of 0.03xc2x0 is required.
Pointing errors occur during operation of satellites. Generally speaking, a pointing error is the angular difference between the theoretical position of the antenna (and/or its reflector) and its actual position on each axis of a three-dimensional system of axes.
Pointing errors are linked in particular with the angular instability of the position of the satellite, with errors in the position of the antenna relative to the satellite, and with internal deformation of the antenna, such as thermal deformation of the reflector. The first two sources of error are the dominant ones and lead to an overall pointing error for all the spots formed by the antenna.
The satellite has attitude control systems, but these achieve accuracy of the order of only one tenth of a degree, which is insufficient with geosynchronous satellites in which the coverage is provided by multiple narrow beams. The antenna must therefore have its own repainting system.
The array antennas used on board satellites can be of two main types, both of which are well known to the person skilled in the art: direct radiation antennas and reflector antennas.
With direct radiation antennas, there is a simple analytical model of the signal received by the elements of the array. The phase of the signals received by the radiating elements is directly related to the direction of arrival of the incident signal. The beam is repointed by in-phase addition of the signals received by the various radiating elements and coming from the required pointing direction. In the same manner, repainting is therefore effected simply as a function of the measured or estimated pointing error, by adding the phase which corresponds to the pointing error to the phase applied by the nominal law.
In contrast, with reflector antennas, the received signal cannot be expressed in a simple analytical form, i.e. there is no direct relationship between the required pointing and the radiating element feed laws.
A mechanical solution is currently envisaged for correcting the pointing error of reflector array antennas: two or three motors control the position of the reflector, which is modified to correct the pointing error, which relates to two or three axes of rotation, as already mentioned.
That solution implies the installation of high precision motors. It is therefore bulky and costly.
Also, modifying the position of the reflector relative to the array changes the configuration of the antenna, which can degrade performance (in particular focusing).
Furthermore, that solution is not sufficiently accurate for large reflectors.
Finally, that solution necessitates the use of additional dedicated antennas and receivers for estimating the pointing error.
The object of the present invention is therefore to provide a method of repainting reflector array antennas that does away with the use of complex, costly, and bulky motors, but nevertheless provides sufficient accuracy, as required by geosynchronous satellites in particular.
To this end, the present invention provides a method of repainting a reflector array antenna comprising a plurality of radiating elements and being of the type that forms beams by computation, in which method each signal received by said antenna is sampled,
said method comprising the following operations:
estimating the depointing of the radiation pattern of said antenna to obtain a phase shift matrix,
computing the discrete inverse Fourier transform of the signal samples supplied by the radiating elements,
multiplying said phase shift matrix by said inverse Fourier transform of said sampled signal, and
computing the discrete direct Fourier transform of the product of said phase shift matrix and said inverse Fourier transform of said sampled signal.
The present invention also provides a method of repainting a reflector array antenna comprising a plurality of radiating elements and being of the type that forms beams by computation, in which method each signal ready to be sent by said antenna is also sampled,
said method comprising the following operations:
estimating the depointing of the radiation pattern of said antenna to obtain a phase shift matrix,
computing the discrete direct Fourier transform of the signal samples to be transmitted by the radiating elements at a given time,
multiplying said phase shift matrix by said direct Fourier transform of said sampled signal, and
computing the discrete inverse Fourier transform of the product of said phase shift matrix and said direct Fourier transform of said sampled signal.
The invention therefore applies a digital correction to the signal sent or received by the antenna, instead of applying a mechanical correction.
The basic idea of the invention relies on the fact that depointing the radiation pattern of the antenna corresponds to a spatial offset (i.e. a phase shift) of the signals received (or sent) by the radiating elements at the focus of the reflector and the fact that, because of the properties of the Fourier transform, offsetting the focal spot in the focal plane of the reflector is converted into simple multiplication by a phase. These operations therefore compute corrections to the signals received or sent by the depointed antenna by simulating the signals of the correctly pointed antenna.
Applying a direct or inverse Fourier transform after multiplication by the phase shift matrix produces signals equivalent to those actually received or sent by the radiating elements of the antenna.
Also, the method of the invention repoints all the beams of a reflector array antenna simultaneously.
The sampling can advantageously be effected after transposing the frequency of the radio frequency signal down to a value in an intermediate frequency band or in baseband.
The depointing is advantageously estimated by a first order digital loop from the known position of at least one fixed beacon.